This invention relates to battery separators comprised of a web formed of a multiplicity of filaments and fibers formed of an ultrahigh molecular weight polyolefin that are microporous and wettable.
By "filament" it is intended to mean an essentially continuous strand of material; by "fiber" it is intended to mean a discontinuous strand of material.
By "strand", within the meaning of this invention, it is intended to mean the product resulting from extruding a molten blend of ultrahigh molecular weight polyolefin, filler, plasticizer and certain minor ingredients through an orifice having a configuration capable of forming a filamentary extrudate, and subsequently extracting all or a part of the plasticizer.
By "microporous" it is intended to mean a filament or fiber that is comprised of a plurality of interconnecting interstices that communicate with the outside and inside of the filament or fiber body, the interstices comprising a void volume of at least twenty percent of said body and preferably at least fifty percent of said body.
By "wettable" it is intended to mean the property of being able to absorb water.
U.S. Pat. No. 4,422,993 discloses spinning solutions of ultrahigh molecular weight polyethylene to form filaments.
U.S. Pat. No. 4,545,950 discloses forming filaments, fibers, etc. by extruding a mixture of ultrahigh molecular weight polyethylene and paraffinic wax, cooling and stretching. The wax may be removed to form a microporous article.
One of the features of this invention is to provide microporous polyolefin filaments and fibers which exhibit good wettability. Polyolefin filaments and fibers are not wettable unless treated. The prior art discloses using wetting agents to impart wettability to polyolefin fibers, such, for example, as disclosed in U.S. Pat. No. 3,870,567; treating nonwoven webs with an aqueous bath comprising water, a surfactant, and colloidal silica such as is disclosed in U.S. Pat. No. 3,985,580; coating nonwoven webs with a hydrophilic vinyl monomer and catalyst such as is disclosed in U.S. Pat. No. 4,110,143; and treatment with corona discharge.